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DDR SDRAM stands for Double Data Rate Synchronous Dynamic Random Access Memory.

It's different from normal SDRAM by the fact it allows reading memory on two channels (rising and falling) and thus it gives theoretical double bandwidth at the same speed.

There are some boards which have DIMMs for both SDRAM and DDR, there aren't any boards that allow both together and your specific board doesn't allow SDRAM at all.

Currently there's DDR, RDRAM and SDRAM, there were other sorts of modules before SDRAM (can't recall their name).

RDRAM currently comes mainly in two speeds, RDRAM 800 and RDRAM 1066, it was only used in P4 platforms and at the time it offered better performance than DDR 266 but at a much higher cost. Currently DDR is the best option both performance and cost wise.